1. Field of the Invention
The present invention relates to a semiconductor substrate capable of preventing deterioration of element characteristics and a method of processing said substrate.
2. Description of the Related Art
In recent years, LSI is widely used in important portions of an electronic computer, a communication equipment, etc. It should be noted that a silicon wafer used for the manufacture of the LSI has a mirror-finished surface.
In preparing a silicon wafer having a mirror-finished surface, a columnar silicon crystal prepared by a CZ method is cut in the first step to obtain a circular silicon wafer. Then, the surface of the silicon wafer thus obtained is lapped, etched or polished to obtain a silicon wafer having a desired thickness. Finally, the resultant silicon wafer is washed with an acidic solution or an organic solvent so as to obtain a desired silicon wafer having a mirror-finished surface.
However, the conventional method outlined above gives rise to serious defects. Specifically, if a silicon wafer is oxidized at a temperature of, for example, about 950.degree. to 1100.degree. C., about 10 to 100 OSF (Oxidation induced Stacking Fault)/cm.sup.2 are formed on the mirror-finished surface. FIG. 10 shows the cause of the OSF formation in a surface region of a silicon wafer 1. Specifically, the OSF formation is considered to be caused by a contaminant 2, a fine scratch 3, a foreign matter 4 such as SiO.sub.2 or SiC, and a micro defect 5 such as a swirl within a silicon wafer 1 or an oxygen precipitates.
If an element is formed in a silicon wafer having such an OSF, a junction leakage takes place so as to deteriorate the element characteristics.
What should also be noted is that, where a columnar silicon crystal is manufactured by the CZ method, an excessive oxygen is dissolved from the crucible into the columnar silicon crystal in the step of the crystal growth. The excessive oxygen causes BMD (Bulk Micro Defect) sized about 0.1 to 1.0 .mu.m.
The BMD is generated in the manufacture of, for example, a CMOS device having a well structure. To be more specific, a p-well or an n-well is formed in a silicon wafer 1 in the initial step of the process for manufacturing a CMOS device of this type. In forming the p-well or n-well, an impurity is thermally diffused into a surface region of the silicon wafer at temperatures not lower than 1100.degree. C. During the thermal diffusion, the oxygen in, particularly, the surface region of the silicon wafer 1 is diffused outward, with the result that a defect-free layer 12 called DZ (Denuded Zone) is formed in the surface region of the silicon wafer as shown in FIG. 11a. Then, the silicon wafer 1 is subjected to heat treatments at a temperature of about 800.degree. C. in various subsequent steps including, for example, the step of forming a silicon nitride film by LPCVD method. During these heat treatments, nuclei 13 of oxygen precipitates are ideally formed in an intermediate layer 14 within the silicon wafer 1, as shown in FIG. 11b. Further, the silicon wafer 1 is subjected to a heat treatment at a temperature of about 1000.degree. C. in the subsequent step of, for example, forming a field oxide film, with the result that precipitate grows about each of the nuclei 13 to form BMD 15 with a high concentration.
The BMD generation described above is greatly affected not only by the heat history of the silicon wafer but also by the carbon concentration, the pull-up condition in the stage of the crystal growth for manufacturing the columnar silicon crystal, etc. Presently, fine oxygen precipitate, which is brought about in the stage of pulling the columnar silicon crystal, is considered to provide the nucleus for the BMD formation. The size and number of precipitates are not uniform within the silicon wafer. Thus, in forming the DZ layer, the oxygen concentration in the surface region of the silicon wafer is lowered as much as possible by the outer diffusion of oxygen such that oxygen is no more precipitated.
The heat treatment for the outer diffusion of oxygen is carried out in general under an oxidizing atmosphere. To be more specific, if a silicon wafer is subjected to a heat treatment under a non-oxidizing gaseous atmosphere, e.g., under a nitrogen gas atmosphere, the wafer surface is nitrided nonuniformly, leading to a surface roughening. Also, in the case of employing an inert gas atmosphere, a non-uniform etching takes place on the wafer surface unless a sufficient purity of the inert gas is ensured. The surface toughening noted above takes place in this case, too. Such being the situation, the heat treatment for the outer diffusion of oxygen is carried out in general under an oxidizing atmosphere, as described above.
However, in the case of the heat treatment under an oxidizing atmosphere, an oxide film is formed on the wafer surface, making it impossible to lower sufficiently the oxygen concentration in the surface region of the wafer. It follows that it is difficult to suppress sufficiently the precipitate generation, resulting in failure to form a satisfactory DZ layer. If a thin oxide film such as a gate oxide film is formed on the surface of a silicon wafer having such an unsatisfactory DZ layer, weak spots are generated so as to lower the breakdown voltage of the oxide film. It may be desirable to form a DZ layer extending deep into the silicon wafer by carrying out the outer diffusion of oxygen over a long time. In this case, however, the concentration of oxygen atoms forming a solid solution within the silicon wafer is lowered, leading to reduction in the mechanical strength of the substrate. Further, crystal defects are generated within the active region of the semiconductor element formed in the wafer.